Process for producing components composed of titanium or titanium alloy by means of mim technology

ABSTRACT

The present invention relates to a process for producing a component composed of titanium or titanium alloy by means of MIM technology. In this process, a homogeneous mixture of boron powder having a particle size of less than 10 μm, preferably less than 5 μm, more preferably less than 2 μm, and titanium powder and/or titanium alloy powder is produced, and binder is mixed with the homogeneous mixture of boron and titanium powder and/or titanium alloy powder and also, if appropriate, an additive in a kneader, the mixture is moulded by injection moulding to produce a green part, the moulded composition is subjected to chemical and/or thermal removal of binder to produce a brown part and the composition from which the binder has been removed is sintered at a temperature in the range from 1000° C. to 1600° C.

FIELD OF THE INVENTION

The present invention relates to a process for producing a component composed of titanium or titanium alloy by means of MIM technology. MIM stands for “metal injection moulding” and is a highly efficient manufacturing process for the production of small, complex and precise metal parts. The MIM technology belongs to the class of powder-metallurgical processes in which not a solid metal body but a fine powder is used as starting material for the component to be produced. This powder is mixed with a polymer-containing binder and kneaded to form the “feedstock”.

BACKGROUND

The feedstock is pressed under pressure into the injection mould (tool) on an injection moulding machine. The green part formed already has the final geometry but, for obtaining a pure metal part, it has to be freed of the binder again in subsequent steps. For this purpose, the binder is removed in a chemical and/or thermal process and the component is “fired” in a sintering step. At present, it is used predominantly for the production of stainless steel components.

Titanium and titanium alloys offer an excellent ratio of strength to weight. These metals are absolutely nonmagnetic, corrosion-resistant and resistant to sea water. In addition, they are biocompatible and are very suitable for implants. This combination of properties leads to the use of titanium in aircraft and spaceflight, marine engineering and medical technology. However, titanium and titanium alloys are very difficult to process.

The use of the MIM technology for producing titanium components is relatively new and restricted essentially to pure titanium. Titanium alloy powders are processed commercially by means of MIM in only a few scattered cases and are restricted to applications which involve only a low change in stress on the component, since the long-term strength is significantly lower than in the case of components produced by cutting machining of a TiAl₆V₄ semifinished part. It is presumed that the existence of pores in the MIM components and a coarser microstructure are responsible for the lower long-term strength of the components produced from titanium alloy powder by means of MIM technology.

SUMMARY

It is an object of the present invention to provide a process for producing components which can be subjected to a high change in stress from titanium or titanium alloy powders by means of MIM.

DETAILED DESCRIPTION

The object is achieved by a process in which a homogeneous mixture of boron powder having a particle size of less than 10 μm, preferably less than 5 μm, more preferably less than 2 μm, and titanium powder and/or titanium alloy powder is produced and binder is mixed with the homogeneous mixture of boron and titanium powder and/or titanium alloy powder and, optionally, an additive in a kneader, the mixture is moulded by injection moulding to produce a green part, the moulded composition is subjected to chemical and/or thermal removal of binder to produce a brown part and the composition from which the binder has been removed is sintered at a temperature in the range from 1000° C. to 1600° C.

The amount of boron powder is preferably selected so that from 0.05% by weight to 1.5% by weight, more preferably from 0.1% by weight to 1.0% by weight, of boron is present in the component, based on the total weight of the latter after sintering. The sintering temperature is preferably in the range from 1000° C. to 1600° C., more preferably from 1200° C. to 1500° C., even more preferably from 1300° C. to 1450° C. Particularly at a temperature in the range from 1300° C. to 1450° C., a residual porosity of the component of less than 3%, based on the component volume, is achieved.

The residual porosity can be determined by measuring the density relative to the density of the solid material or by geometric analysis of polished sections examined under a microscope.

The uptake of oxygen during the process should preferably be limited to such a degree that the sintered components have an oxygen content of less than 0.3% by weight, based on the total weight of the component, since otherwise the ductility of the components is adversely affected. For this reason, the mixing of boron powder and titanium powder and/or titanium alloy powder preferably takes place under a protective gas atmosphere. The mixing of the binder with the homogeneous mixture of boron and titanium powder and/or titanium alloy powder and, optionally, an additive preferably also takes place under a protective gas atmosphere. As protective gas, preference is given to using argon or helium, more preferably argon. Sintering is preferably carried out in a high vacuum. In addition, a getter material such as titanium can be present. The latter measures serve to minimize the oxygen uptake by the brown parts during sintering.

The oxygen content of the sintered component is preferably determined by melt extraction analysis.

Preference is given to using a starting powder which is particularly low in oxygen and a binder which is likewise low in oxygen. The titanium powder and/or titanium alloy powder typically has a particle size of less than 45 μm. As titanium alloy powder, it is possible to use, for example, TiAl₆V₄ which has preferably been produced by inert gas atomization.

The binder is preferably selected from among thermoplastic or thermoset polymers, thermogelling substances, waxes and surface-active substances and mixtures thereof. Preference is given to using polyamides, polyoxymethylene, polycarbonate, styrene-acrylonitrile copolymers, polyimides, natural waxes and/or oils, thermoset resins, cyanates, polypropylenes, polyacetates, polyethylenes, ethylene-vinyl acetate copolymers, polyvinyl alcohols, polyvinyl chlorides, polystyrene, polymethyl methacrylates, anilines, mineral oils, agar, glycerol, polyvinyl butyrals, polybutyl methacrylates, cellulose, oil acids, phthalates, paraffin waxes, carnauba wax, ammonium polyacrylates, diglyceride stearates and diglyceride oleates, glyceryl monostearate, isopropyl titanates, lithium stearate, monoglycerides, formaldehyde, acid octyl phosphates, olefin sulphonates, phosphate esters or stearic acid or mixtures thereof as binder. The binder particularly preferably contains polyethylene, stearic acid, paraffin and carnauba wax. The binder most preferably contains a polyethylene copolymer such as polyethylene-vinyl acetate copolymer (PEVA) or polyethylene-butyl methylacrylate copolymer (PBMA) and also paraffin.

The green part is, in step (d), subjected to chemical binder removal in a hydrocarbon, preferably hexane and/or heptane, and preferably subsequently subjected to thermal binder removal at a temperature of preferably from 300° C. to 600° C., more preferably from 400° C. to 500° C. The chemical binder removal usually takes place at temperatures in the range from ambient temperature to 60° C., preferably from 40° C. to 50° C.

The invention will now be illustrated by the following, nonlimiting example. The particle sizes are, unless indicated otherwise, maximum particle sizes. The titanium alloy powder used was obtained by sieving.

EXAMPLE

Gas-atomized spherical powder having the composition corresponding to ASTM grade 23 (TiAl₆V₄ ELI) having a particle size of less than 45 μm is used as starting material. This is homogeneously mixed under an argon atmosphere with an amorphous boron powder having a particle size of less than 2 μm. The powder mixture is then kneaded still under an argon atmosphere with the binder constituents PEVA and paraffin in a Z blade mixer at a temperature of 120° C. for 2 hours to form the feedstock and pelletized.

The feedstock is processed on an Arburg 320S injection moulding machine at a melt temperature in the range from 100° C. to 160° C. to produce test specimens (here bars for tensile tests). The green parts are subjected to chemical binder removal in heptane at 40° C. for 20 hours, which dissolves out the wax component of the binder system. The brown parts are placed in a high vacuum furnace with ceramic-free lining and tungsten heater.

The residual binder is firstly thermally decomposed under an argon atmosphere in the furnace by means of a suitable temperature programme and drawn off by means of a vacuum pump before the sintering of the metal powder is carried out directly afterwards. Sintering preferably takes place under reduced pressure at a pressure of 10−4 mbar. The sintering temperature is typically 1400° C., and the sintering time is 2 hours.

The mechanical properties measured on the sintered parts are shown by way of example for the use of TiAl₆V₄ ELI powder in the following table, once without and once with addition of 0.5% by weight of boron. Comparison is made with the standard for the corresponding material as mechanical alloy:

Yield Tensile Long-term strength Alloy point [MPa] strength [MPa] Elongation [%] [MPa] MIM-Ti-6Al-4V 757 861 14 450 (comparison) MIM-Ti-6Al-4V- 790 902 11 640 0.5B (invention) Ti-6Al-4V Grade 23 759 828 min. 10  500* (comparison) *Alpha-lamellae, width 12 μm, heat-treated state 

1. Process for producing a component composed of titanium or titanium alloy, comprising: (a) mixing a boron powder having a particle size of less than 10 μm and titanium powder and/or titanium alloy powder to produce a homogeneous boron powder with titanium powder and/or titanium alloy powder, (b) mixing a binder with the homogeneous mixture of boron and titanium powder and/or titanium alloy powder and, optionally, an additive, in a kneader, (c) molding the mixture of (b) by injection molding to produce a green part, (d) subjecting the green part to chemical and/or thermal removal of the binder to produce a brown part, and (e) sintering the brown part at a temperature in the range from 1000° C. to 1600° C.
 2. Process according to claim 1, wherein the amount of boron powder is selected so that from 0.05% by weight to 1.5% by weight of boron is present in the component, based on the weight of the component after sintering.
 3. Process according to claim 2, wherein the boron content is in the range from 0.1% by weight to 1.0% by weight.
 4. Process according to claim 1, wherein the sintering temperature is in the range from 1300° C. to 1450° C.
 5. Process according to claim 1, wherein the mixing of boron powder and titanium powder and/or titanium alloy powder takes place under a protective gas atmosphere.
 6. Process according to claim 1, wherein the mixing of binder with the homogeneous mixture of boron powder and titanium powder and/or titanium alloy powder takes place under a protective gas atmosphere.
 7. Process according to claim 5, wherein the protective gas is argon or helium.
 8. Process according to claim 1, wherein sintering takes place in a vacuum.
 9. Process according to claim 1, wherein the sintered components have an oxygen content of less than 0.3% by weight, determined by melt extraction analysis.
 10. Process according to claim 1, wherein the titanium powder and/or titanium alloy powder has a particle size of less than 45 μm.
 11. Process according to claim 1, wherein TiAl₆V₄ is used as titanium alloy powder.
 12. Process according to claim 11, wherein gas-atomized TiAl₆V₄ is used.
 13. Process according to claim 1, wherein the binder is selected from the group consisting of thermoplastic or thermoset polymers, thermogelling substances, waxes and surface-active substances and mixtures thereof.
 14. Process according to claim wherein the green part is, in step (d), subjected to chemical binder removal in a hydrocarbon, to produce a brown part.
 15. A component composed of titanium or titanium alloy, produced by the process of claim
 1. 16. Process according to claim 6, wherein the protective gas is argon or helium.
 17. Process according to claim 14, wherein the hydrocarbon is hexane or heptanes.
 18. A component composed of titanium or titanium allow produced by the process of claim
 2. 19. The component of claim 15, wherein the component is composed of a titanium alloy.
 20. The component of claim 19, wherein the titanium alloy is TiAl₆V₄. 